1. Technical Field
The present invention relates to a circuit board, a mounting structure of a ball grid array, an electro-optic device, and an electronic device. More particularly, the present invention relates to a circuit board, a mounting structure of a ball grid array, an electro-optic device, and an electronic device, in which the printing margin of a soldering material is increased upon mounting the ball grid array on the circuit board by reflow processing.
2. Related Art
According to a widespread conventional method, a QFP (Quad Flat Package) is mounted on a printed circuit board (PCB) as a mounting method corresponding to the fine pitch or the multi-pin structure of a lead terminal of a semiconductor device. According to the method, the QFP as a flat package having a large number of gull-wing lead terminals at four sides of the package is generally mounted on the PCB containing resin. Further, the QFP is used while it is electrically connected to a conductive portion of the PCB.
However, in accordance with the further fine pitch or multi-pin structure, the QFP has a problem that the reliability of the connection is deteriorated due to a short circuit caused by a soldering bridge upon mounting or the shortage of soldering. In addition, the QFP has another problem that the mounting area on the PCB is increased corresponding to an externally projected portion of the lead terminal from the package.
Then, in order to respond to the further fine pitch or multi-pin structure of the semiconductor device, a mounting method using a ball grid array (hereinafter, referred to a BGA) or a chip size package (hereinafter, referred to as CSP) and a printed circuit board for mounting it are proposed.
Referring to FIG. 25, a single printed-circuit board has semiconductor package lands 382 and 384 having different shapes and, on the printed-circuit board, lands to which terminals having the same function are connected by a pattern wiring 388. If changing the shape of the package mounted on the printed circuit board, the same printed-circuit board is used without manufacturing another printed-circuit board.
Further, according to another mounting method, an electric part in which the exchange and the maintenance of the BGA, etc. are not possible is soldered so as to prevent the soldering defect. As schematically shown in a flowchart of FIG. 26, specifically, a mounting method 290 includes: a step 291 of printing cream soldering on a predetermined position on the PCB; a step 292 of mounting, by a mounter, a semiconductor device with a bump such as the BGA (including a chip size package (CSP) with a pitch of not more than 0.8 mm) at a predetermined position on the PCB to which the cream soldering is printed; a step 293 of performing an X-ray inspection and of selecting whether or not the semiconductor device is defective; and a step 294 of heating only the nondefective semiconductor device passing the X-ray inspection by reflow processing and mounting.
Further, referring to FIGS. 27(a) to (d), according to another mounting method of the BGA, a cream soldering 312 is coated to a cave (recessed) portion 311 arranged on a board 310.
Specifically, referring to FIG. 27(a), the board 310 having a plurality of cave portions 311 on its surface is provided. Referring to FIG. 27(b), the cream soldering 312 is coated to the plurality of cave portions 311. Furthermore, referring to FIG. 27(c), a bump 317 of a BGA 315 is positioned to the plurality of cave portions 311. In addition, referring to FIG. 27(d), the bump 317 is mounted by reflow processing and is pressed and bonded.
According to another method, referring to FIGS. 28(a) to (c), a semiconductor device 345 with a bump is thermally bonded to a pad 341 of a board 343 via an Anisotropic Conductive Film (hereinafter, referred to as an ACF) 349.
According to the mounting method using the ACF 349, in the CSP in which the pitch of the bump 347 is narrow, for example, it is 0.1 mm to 0.5 mm, the short circuit between the adjacent bumps is efficiently prevented. Advantageously, a large number of bumps 347 are electrically connected.
However, on the convention printed-circuit board, upon mounting the semiconductor device with a fine bump, e.g., the BGA, a soldering material as a conductive material must be precisely printed onto a fine land (pad). Thus, the conventional printed-circuit board has a problem that the position for printing and the printing operation take a long time and the position of the printed soldering material is easily deviated from the pad. In particular, since the CSP has a further fine pitch as compared with the BGA, it is difficult to print the soldering material precisely and mount it on the land (pad) of the printed circuit board on which the FPC is easily modified.
According to the mounting method as shown in FIG. 26, the X-ray inspection must be performed before the heating using reflow processing. Therefore, the mounting method has a problem that the number of steps is increased, the manufacturing management is complicated, and the manufacturing time is long. Since the cream soldering must precisely be printed on the fine pad, the positioning for printing and the printing operation take a long time.
According to the mounting method as shown in FIG. 27, it is difficult to form the pad having the cave portion. Further, since the soldering must be precisely printed to the pad having the fine cave portion, the mounting method has a problem that the positioning for printing and the printing operation take a long time.
Further, the mounting method using the ACF has problems that not only the costs of the ACF are increased but also mounting simultaneously to another device is not performed. That is, the ACF which is thermally bonded and mounted, and the other device mounted by the soldering reflow processing must be mounted independently in consideration of the different mounting processes.
As a result of considering the above problems, it has been discovered that by providing a soldering resist having an opening portion which is open so that the entire or a part of the pad and wiring connected to the BGA is exposed, the coating margin (printing margin) of the soldering material is increased, the printing deviation due to the setting of an uneven portion of the resist is not caused, and thus the soldering material is coated precisely and easily. Further, the defect for connection is caused due to the deviation of printing of the soldering material. In this case, it is found that the soldering material out of the pad due to the printing deviation returns to the pad upon reflowing by its fluid property, irrespective of the deviation or uneven portion of the resist.
As such, it is an object of the present invention to provide a circuit board in which the mounting position of a BGA (including the CSP) is not deviated due to the coating defect of the soldering material when the BGA is mounted by reflow processing via the soldering material, a mounting structure of the BGA using the circuit board, an electro-optic device, and an electronic device.